Pearring Lab at the University of Michigan

Our laboratory studies the molecular and cellular mechanisms of vision. Our work focuses on the vertebrate photoreceptor cell, a polarized neuron responsible for detecting light that enter the eye. In these studies, we utilize both lower and higher vertebrate animal models, thereby taking advantage of methodologies applicable to each animal type. Many of our experiments are conducted in transgenic Xenopus frogs, whose photoreceptors are very large so uniquely suitable for morphological and live imaging studies. Other experiments are performed with mouse photoreceptors, which allow us to employ many genetically manipulated mouse lines and to use another high throughput gene delivery technique of in vivo electroporation. 

We are currently pursuing two main directions. 

Protein delivery to the light-sensing outer segment organelle of photoreceptors. 

Understanding the underlying mechanisms that establish and maintain the subcellular distribution of proteins remains a central unsolved problem in cell biology. Photoreceptors serve as a productive model for studying this phenomenon since they are highly polarized neurons with a clearly defined function and a wealth of biological techniques developed for their study. Additionally, defects in protein trafficking and mislocalization in photoreceptors underlie many form of inherited retinal degenerative diseases. Our lab aims to understand the basic cell biological mechanisms employed by photoreceptors to ensure polarized delivery of signaling and structural proteins to the light-sensitive outer segment compartment. Current projects include:

How are proteins segregated between the outer segment membrane subdomains: the discs and ciliary plasma membrane?     The light-sensing ability of photoreceptors depends on the localization of different signaling proteins to these specific subdomains. We are exploring the molecular mechanisms guiding delivery of the cyclic nucleotide gated channel to the ciliary plasma membrane and how this is similar and/or different from disc-specific protein delivery.

How are membrane proteins deposited into the light-sensing outer segment compartment of photoreceptors?     Recent evidence from the ciliary and retinal cell biology fields have implicated small GTPases (Rho, Ral and Rab families), transition zone proteins (cc2d2a, NINL, MICAL3), SNARE proteins (Syntaxin 3 and SNAP25) and molecular motors (dynein and myosin V) in docking and fusing secretory vesicles at the base of the cilia. Using in vivo model systems, we are exploring whether these candidates play a role in trafficking to or formation of the ciliary outer segment compartment.

Biogenesis of the photoreceptor outer segment organelle

Photoreceptor outer segments are cylindrical structures filled with hundreds of flattened membrane discs. This structural organization provides membrane stacks to densely pack with visual pigment molecules, allowing for highly efficient light capture. Throughout the lifespan of an animal, discs undergo continuous renewal with new membranes added at the outer segment base and old membranes shed from the tip. Interestingly, the structural organization of outer segments differs between rod and cone photoreceptors. Highly sensitive rod cells have “closed discs” that are physically and electrically separate from the ciliary plasma membrane, while cones have “open discs” that are contiguous in-foldings of the ciliary membrane. In spite of these structural differences, both rods and cones form new discs through serial evagination of the ciliary membrane. However, the molecular mechanism of disc formation remains elusive. Current project includes:

How are the subdomains of rod outer segments formed: separation of discs from the ciliary plasma membrane?     Rod photoreceptors are responsible for vision under dim-light levels. One-way rods boost their sensitivity is to separate the outer segment disc membranes from the ciliary plasma membrane, which increases the cytosolic space for second messenger diffusion and reduces the overall membrane capacitance. What is not understood is how the newly forming ciliary membrane evaginations are separated to forms these two distinct membrane subdomains. 

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